Transition in a two-dimensional plane wall jet

Abstract

Transition in a two-dimensional plane wall jet was examined for the range of exit Reynolds numbers between 100 and 600. The wall jet was produced by the flow of water, which issued either from a smoothly-contoured nozzle or from the end of a long parallel channel. Observations were recorded both visually and with a hot-film anemometer. Both natural and forced transition cases were studied. Natural transition generally occurred in the following stages: (i) formation of discrete vortices in the outer shear layer; (ii) coalescence of adjacent vortices in the outer shear layer, coupled with the rolling up of the inner shear layer; (iii) eruption of the wall jet off the surface of the flat plate into the ambient fluid; (iv) dispersion of the organized flow pattern by three-dimensional turbulent motions; and (v) re-laminarization of the upstream flow, until another vortex pairing occurred. The initial stages of transition are two-dimensional in nature; and are dominated by the mechanism of vortex pairing, which is commonly observed in free shear layers. The inner-region shear layer exerts a stabilizing influence on the velocity profile at low Reynolds numbers. Forced transition was essentially similar to natural transition, except for the elimination of the downstream intermittency and the establishment of a fixed downstream location for transition.